Method of fabricating a matrix cathode



March 8, 1966 c, RICH ETAL 3,238,596

METHOD OF FABRICATING A MATRIX CATHODE Filed Oct. 23, 1962 INVENTORS CHARLES E. R/CH $1. MbM

ATTORNEY United States Patent 3,238,596 METHOD OF FABRHCATING A MATRIX CATHODE Charles E. Rich and Charles K. Trace, Gainesville, Fla., assignors to Sperry Rand Corporation, Great Neck, N.Y., a corporation of Delaware Filed Oct. 23, 1962, Ser. No. 232,540 5 Claims. (Cl. 2925.17)

This invention relates to a method of fabricating a matrix cathode for use in klystrons and in traveling wave tubes, and more particularly relates to a simple method that produces rugged cathodes having high current emission and long life.

A great many different process and combinations of materials are known for producing matrix cathodes for use in electron beam tubes. Many of these processes require close control of the processing steps and conditions, and in many instances the .completed cathode is relatively easily damaged in operation due to sparking, back-ion bombardment and poisoning of the emiss-ive material or of the material from which the emissive material is derived. 7

It is an object of this invention tonprovide a rugged, long-life matrix cathode.

It is another object of this invention to provide a means for producing a matrix cathode that possesses mechanical, thermal, and electrical excellence.

It is a further object of this invention to provide a rugged high current matrix cathode that is easily produced with conventional facilities and readily-available materials.

In accordance with the present invention a nickel powder and a small amount of an activating material such as titanium hydride are mixed in asuitable binder or liquid vehicle that will be driven off or will decompose into a non-carbonaceous residue upon heating. This mixture is deposited upon a cathode backing surface of a material such as nickel. The mixture of nickel powder and activating material then is sintered to form a porous nickel matrix having the activating material bound therein. A composition of alkaline earth carbonate materials then is distributed substantially uniformly throughout the porous nickel matrix. The matrix then is subjected to a very high compressive force to compact the matrix to approximately twice its former density and to impart the desired shaped surface thereto. A further coating of alkaline earth carbonate material then is applied to the shaped surface, and the carbonate material is permitted to dry. The cathode then may be stored for future use without danger of contamination or adverse efifect on the constituent materials, or the cathode may be activated for immediate use.

The method of this invention will be described by referring to the accompanying drawings wherein:

FIG. 1 illustrates an early step of the method wherein a mixture of nickel powder and an activating material is deposited on a cathode button;

FIG. 2 is a simplified sketch illustrating the step of compressing the impregnated nickel matrix; and

FIG. 3 is a sketch of a completed cathode produced in accordance with the teachings of this invention.

Referring now in more detail to the individual steps of this invention, a mixture is prepared of a nickel powder and the activating material titanium hydride powder wherein the titanium hydride forms approximately one percent by weight of the mixture. This percentage is not critical and may range from .25 to 1.5 percent. These two materials are mixed in a binder material or a liquid vehicle so as to form a smooth slurry or paste. In a typical example of such a mixture, we 'have used a carbonyl nickel powder having a particle size between seven and nine microns, obtainable from SKC Research Associates, Paterson, New Jersey. The particle size of this nickel powder is not considered to be critical and may range between 3 and 12 microns and still produce acceptable results. The titanium hydride powder has a particle size of approximately 44 microns, and is obtainable from Metal Hydrides Company, Beverly, Massachusetts. The particle size of the titanium hydride is not critical. For the range of nickel powder particles given above, the titanium hydride may range between 25 and 45 microns. The hydride of zirconium also may be .used as an. activating material. It is important that the liquid vehicle or some other mixing binder it used, be of a substance that does not decompose toa carbonaceous residue when the mixture is subjected to a sintering process. A carbonaceous residue is very undesirable because it may produce initial overactivity in the conversion of the alkaline earth carbonate material, and thus shorten the life of the cathode. Additionally, the carbonresidue can contribute to the formation of carbon monoxide which will cause a sealed-01f tube to become gassy.

In accordance with. this purpose, we use an alcohol mixture such as C. P. Methanol, purchased from Fisher Scientific Co., Fairlawn, New Jersey, with the addition of 10% Dytol L79, a fatty alcohol consisting of a minimum of 98% lauryl alcohol with the remainder being a maximum of 2% decyl alcohol and/or a maximum of 2% myristyl alcohol, purchased from Rohm and Haas Chemical Corporation, Philadelphia, Pennsylvania. This alcohol mixture is driven off at a low temperature in the early stages of the sintering process, and thus will not adversely react with or contaminate the nickel and titanium hydride. The amount of mixing binder or vehicle used is dependent upon the mode of application of the mixture to a cathode backing surface. Other suitable binder material or liquid vehicle such as C. P. Methanol with the addition of 10% amyl acetate also may be used.

A suspension of the two materials prepared in the above-described manner next is deposited on a suitable cathode backing material such as the spherically concave cathode button 10, FIG. 1, to form a porous, loosely-compacted coating 11. Cathode button 10 commonly is made of cathonic nickel. In one cathode made by the method of this invention wherein the cathode button was 1.697 inches in diameter, the mixture of nickel powder and titanium hydride was applied in a coating .005 inch thick. Cathode button 10 with the coating 11 of nickel powder and titanium hydride powder thereon then is heated in an oven to a temperature of apprixmately 1000 C. for approximately twenty minutes to substantially completely sinter the nickel powder. This heating is carried out in a non-oxidizing atmosphere, such as a mixture or dry hydrogen and nitrogen. The sintered nickel matrix now is a rigid body comprised of from 50 to 60 percent void space.

Alkaline earth carbonate materials such as the carbonates of barium, strontium and calcium, either singly or in any combination, then are distributed throughout the porous nickel matrix so as to fill approximately of the void spaces in the matrix. In practice, we prepare a methanol suspension of a mixture of the commercially available material RCA-33C130 which consists of coprecipitated barium, strontium, and calcium carbonates in the proportions 57:39:4, respectively, and obtainable from Radio Corporation of America, Harrison, New Jersey. This suspension of material is soaked into the porous matrix to the extent described above. After the soaking action is completed the matrix is dried in a low temperature oven to drive off the alcohol and moisture so as to leave the emissive material impregnated in the nickel matrix. Care should be taken during this heating to prevent thermal decomposition of the carbonates. Heating at a temperature below 200 C. will substantially assure that the carbonates will not convert to oxides.

The cathode button next is inserted over a mandrel 12, FIG. 2, and a tubular sleeve 13 is inserted thereover to hold the cathode button 10 securely in position. Next, a compressing tool is inserted within sleeve 13 and a compressive force F of approximately 40 tons per square inch is exerted on compressing tool 15 to compact the sintered matrix 11 having the emissive material distributed throughout. Compressing tool 15 has a shaped surface 16 at its lower end to impart the desired shape to the mixture of powder 11.

The compacting of the nickel matrix greatly improves the operation of a cathode because its thermal conductivity is increased, thus reducing the heater power require: ments and assuring uniform heating. Further, the compacting assures intimate contact between the metallic matrix and the emissive materials. This forms an extended interface, thus lowering the interface resistance. This compressing further makes the titanium hydride activating material readily available to react uniformly with the carbonates in the known manner throughout the life of the cathode.

The addition of carbonate materials to the nickel matrix after it has been sintered is considered to be an important feature that contributes to the long life of the cathodes made by the method of this invention, and also contributes to the high yield of acceptable cathodes made in accordance with this method. We have found that it is undesirable to have the carbonates present in the initial mixture of nickel powder and titanium hydride powder because the carbonates have a tendency to convert to oxides during the sintering steps. The oxides will hydrolize when exposed to humid air and this causes expansion and distortion of the electron emissive surface and results in an unacceptable cathode. On the other hand, adding the carbonate materials to the already-sintered nickel matrix and compressing the matrix, as in the present invention, leaves the carbonate materials as very stable compounds since they have not as yet been subjected to conditions that would tend to convert them. This is quite desirable because the cathodes at this stage of production, prior to activation, will not require special handling or elaborate storage facilities since they will not be affected by the atmospheric conditions usually found in a production plant. This, of course, reduces the complexity and cost of production and produces higher yields of acceptable cathodes. Also, the cathodes need not be used immediately but may be stored for future use with a minimum of special care. The compacting of the matrix under high pressure improves the emission stability, and reduces the possibility of sparking under high current density and high voltage conditions.

The cathode may be activated in its present condition, or it may be activated after a thin coating of from 4 to 6 milligrams per square centimeter of the alkaline earth carbonate material has been sprayed onto the emitting surface and allowed to dry. A cathode of the latter type is illustrated in FIG. 3 wherein the sintered matrix 11 uniformly covers the spherical surface of cathode button 10 and a thin coating of the carbonate material is deposited on the emitting surface of matrix 11.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

4 What is claimed is: 1. The method of forming an impregnated matrix cathode comprising the steps of,

forming 'a mixture of nickel powder and an activating 5 agent chosen from hydrides of titanium or zirconiu-m wherein said activating agent ranges from .25 to 1.5 percent by weight of said mixture, sintering said mixture to form a loosely compacted sintered matrix, impregnating said matrix with a material which when suitably processed freely emits electrons when heated,

compressing said impregnated matrix to more tightly compact said matrix.

2. The method claimed in claim 1 wherein,

said nickel powder and activating agent are admixed in a mixing vehicle which leaves no carbonous residue in said mixture when the mixture is sintered.

3. A method for constructing an impregnated matrix cathode comprising the steps of,

preparing a homogenous mixture of the powders of metallic nickel and the hydride of titanium or zirconium, wherein the hydride ranges from .25 to 1.5 percent by'weight of said mixture,

applying said mixture to a cathode backing surface to make a thin uniform coating thereon,

sintering said mixture of powders to form a loosely compacted sintered matrix on said cathode backing surface,

impregnating said matrix with a material selected from the carbonates of barium, strontium and calcium to fill a major portion of the void spaces of the loosely-compacted matrix,

compressing said loosely compacted matrix with the carbonate material therein to substantially reduce the volume of said matrix and thereby produce a compacted impregnated cathode matrix suitable for activation and use in an electron discharge device.

4. The method of making a rugged electron emissive cathode having a long life comprising the steps of,

preparing a homogenous mixture of nickel powder and an activating material selected from the hydride of titanium or zirconium, wherein the hydride ranges from .25 to 1.5 percent of said mixture,

said mixture including a mixing vehicle that will leave no carbonous residue when heated,

applying said mixture to a cathode backing surface to form a loosely compacted deposit on said surface, sintering said deposit of mixed powders in a non-oxidizmg atmosphere to substantially completely sinter the powders thereby forming a rigid, loosely compacted matrix,

impregnating said loosely compacted matrix with material selected from the carbonates of barium, strontiumand calcium to fill a major portion of the remammg volume of said loosely compacted matrix,

applying a compressive tool to the surface of said impregnated matrix to reduce its volume by approximately one-half and to impart a desired shape-d surface to the matrix, and

applying a thin coating of said impregnating material to the shaped surface of said matrix.

5. The method of making a rugged electron emissive 65 cathode having a long life comprising the steps of,

forming a suspension of nickel powder having a particle size ranging from 3 to 12 microns and titanium hydride powder having a particle size ranging from to 45 microns in a liquid vehicle that is characterized by thermally decomposing to a non-carbonaceous residue, applying said suspension to a cathode backing surface to form a loosely compacted deposit of said materials, heating said cathode backing surface and deposited materials thereon in an oven having a non-oxidizing atmosphere to substantially completely sinter said powders to form a rigid, loosely compacted matrix comprised of from 40 to 60% void spaces,

impregnating said matrix with material selected from the carbonates of barium, strontium and calcium to fill approximately 75% of the void space in said matrix,

applying a compressive tool to the exposed surface of said matrix with suflicient force to reduce the thickness of said matrix by approximately 50% and,

applying a coating of from four to six milligrams per square centimeter of said carbonate materials to the top surface of said compacted matrix.

References Cited by the Examiner 5 UNITED STATES PATENTS 2,945,150 7/1960 De Santis 29-25.17 2,996,795 8/1961 Stout 29195 3,088,851 5/1963 Lemmens 2925.17 3,117,249 1/1964 Winters -2 2925.17

10 RICHARD H. EANES, JR., Primary Examiner. 

1. THE METHOD OF FORMING AN IMPREGNATED MATRIX CATHODE COMPRISING THE STEPS OF, FORMING A MIXTURE OF NICKEL POWDER AND AN ACTIVATING AGENT CHOSEN FROM HDYRISES OF TITANIUM OR ZIRCONIUM WHEREIN SAID ACTIVATING AGENT RANGES FROM .25 TO 1.5 PERCENT BY WEIGHT OF SAID MIXTURE, SINTERING SAID MIXTURE TO FORM A LOOSELY COMPACTED SINTERED MATRIX, IMPREGNATING SAID MATRIX WITH A MATERIAL WHICH WHEN SUITABLY PROCESSED FREELY EMITS ELECTRONS WHEN HEATED, COMPRESSING SAID IMPREGNATED MATRIX TO MORE TIGHTLY COMPACT SAID MATRIX. 